I read about Observer pattern in the GoF book.
.Net Framework contains the INotifyPropertyChanged interface. Has .Net Framework the similar interface but for notifying about changing of the set of properties instead of notifying about each property changed (for increase in productivity)?
I created such interface INotifySubjectChanged for my application (its event argument contains the set of changed properties) but maybe .Net Framework already has it and I invented a bicycle.
I published my simple code source on Bitbucket.org here. It maybe more clear what I want to do.
MSDN states that leaving the property name blank implies that all of the properties on the object have changed.
https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged.propertychanged(v=vs.110).aspx
The PropertyChanged event can indicate all properties on the object have changed by using either null or String.Empty as the property name in the PropertyChangedEventArgs.
Which is probably as close as you could get to what you are asking for from the .NET framework. Note that using this incorrectly and calling it when a smaller number of properties have changed might actually reduce performance in certain situations.
For record if the object is a collection (which you said it is not) you could leverage INotifyCollectionChanged.
If you are concerned about performance and efficiency, I'd first make sure that you actually are having performance issues and that the notification of changes is causing those performance issues.
If you are not concerned about your implementation working with WPF data binding and it is relatively clear as to its behavior, then I don't see anything wrong with the custom interface and event.
Related
I am trying to write an analysis tool that compares and tracks data over time in your application.
My goal currently is to find a way to send a trigger to a manager, once the value of a tracked property changes.
This process of setting up tracked properties should be as simple as possible, in an ideal world without the need of changes on the actual property code-wise, like the INotifyPropertyChanged interface. Any property with a valid getter&setter should be trackable.
My idea was to use Reflection to hook into the setter method of the property, and add a callback there once the setter is called.
Is there a better way to go about this? Or can reflection manage this, and if yes, how?
If triggers should be sent as soon as possible on every change, the only option is to add wrappers around properties (one way or the other). AOP frameworks can significantly simplify this and help to avoid boilerplate code (e.g. INotifyPropertyChanged "pattern"). There are a few of them each with its pros and cons, for example:
PostSharp
AspectInjector (an example of what may be helpful for your needs
https://github.com/pamidur/aspect-injector/tree/master/samples/NotifyPropertyChanged)
Using Protobuf-net, I want to know what properties of an object have been updated at the end of a merge operation so that I can notify interested code to update other components that may relate to those updated properties.
I noticed that there are a few different types of properties/methods I can add which will help me serialize selectively (Specified and ShouldSerialize). I noticed in MemberSpecifiedDecorator that the ‘read’ method will set the specified property to true when it reads. However, even if I add specified properties for each field, I’d have to check each one (and update code when new properties were added)
My current plan is to create a custom SerializationContext.context object, and then detect that during the desearalization process – and update a list of members. However… there are quite a few places in the code I need to touch to do that, and I’d rather do it using an existing system if possible.
It is much more desirable to get a list of updated member information. I realize that due to walking down an object graph that may result in many members, but in my use case I’m not merging complex objects, just simple POCO’s with value type properties.
Getting a delta log isn't an inbuilt feature, partly because of the complexity when it comes to complex models, as you note. The Specified trick would work, although this isn't the purpose it was designed for - but to avoid adding complexity to your own code,that would be something best handled via reflection, perhaps using the Expression API for performance. Another approach might be to use a ProtoReader to know in advance which fields will be touched, but that demands an understanding of the field-number/member map (which can be queried via RuntimeTypeModel).
Are you using habd-crafted models? Or are you using protogen? Yet another option would be to have code in the setters that logs changes somewhere. I don't think protogen currently emits partial method hooks, but it possibly could.
But let me turn this around: it isn't a feature that is built in right now, and it is somewhat limited due to complexity anyway, but: what would a "good" API for this look like to you?
As a side note: this isn't really a common features in serializers - you'd have very similar challenges in any mainstream serializer that I can think of.
I was working with angular-js and came across interesting feature that can watch object for changes.
I wonder if there is something similar in c#. I thought to create some watcher that will accept object to be watched, and may be check the hash code of the object periodically to find out if object changed, then rise an event. But that seems a bit inefficient.
Any ideas?
Thanks
That wouldn't work anyway, firstly because there is no requirement for a property change to affect the hash code, and secondly, even if it did affect the hash code, it would be possible for the new hash code to be equal to the old hash code.
What AngularJS does is keep track of the last-known values, and then re-load all of the values, comparing them to the prior ones. If they're no longer equal, there has been a change.
You could do the same in C# if you want to.
But a way that's more common in C# is for the object being watched to implement the INotifyPropertyChanged interface, and raise the PropertyChanged event on each property change. This gives the object the ability to notify any interested watchers that properties have changed, without requiring the watcher to do any polling.
In both JavaScript and C#, objects aren't observable per se.
Actually, Angular turns objects into observable objects under the hoods, and since JavaScript is a dynamically-typed language this conversion or wrapping can be done in a painless way.
Since C# is a strongly-typed language, you can't observe objects if they don't implement an interface or something that can expose an event to subscribe to changes. Usually observable objects should implement INotifyPropertyChanged interface.
Other way of implementing this is using a framework like Castle DynamicProxy to create proxies of your classes and intercept both method calls and property gets and sets.
I've found many examples of DynamicProxy used to perform additional work when property changes in the object, but how do I not only set the property value in the interceptor (by calling Proceed()), but also roll it back, if something went wrong during interception itself?
Usecase here is this: I use settings objects to provide setting values to some parts of the system, so they won't have to work with the configuration backend.
These objects are also INotifyPropertyChanged, so if anyone changes the setting at runtime, the component which has the object gets notified about the change and can perform reconfiguration on the fly.
So what I need to do is roll back the change of the property if PropertyChanged event handler has thrown an exception (the change led to misconfiguration or was invalid).
I wonder how it can be done.
I've ended up using both INotifyPropertyChanging (and extended this interface a bit) and INotifyPropertyChanged. Simply because the validation should be performed before the actual setting, because
order of events is not determined, and so the actual consumer of the setting can get the notification before it has been even validated, when you using only INPChanged
and overall the validation and consuming are different and should have different events: one before the setting took place (to prevent it from happening) and one after - when the actual value of the property is already updated (for the code to work as expected).
When I look at this now, the original design (which the question is about) seems bad to me.
I'm primarily an Objective-C/Cocoa developer, but I'm trying to implement the Observer pattern in C#.NET, specifically mimicking the NSKeyValueObserving protocols and methodology.
I've gotten as far as mimicking NSKVO with manual support, as described in Apple's KVO Programming Guide (see http://tinyurl.com/nugolr). Since I'm writing the setValue:forKey: methods myself, I can implement auto KVO notification through there.
However, I'd like to somehow implement auto KVO on all properties by dynamically overriding them at runtime. For example, replacing Button.Title.set with:
set {
this.willChangeValueForKey("title");
title = value;
this.didChangeValueForKey("title");
}
So, this is my question:
How do I dynamically override a method or property at runtime in C#? I've gotten as far as getting and invoking methods and properties by name using Reflection.MethodInfo. Alternatively, can I observe the runtime and find out when a method is about to be/has been called?
Dynamic metaprogramming and aspect oriented programming are not yet strongly supported in C#. What you can do, is look at a free tool called PostSharp - it allows supports weaving aspects into your code around properties and method calls quite easily.
You can implement the INotifyPropertyChanged interface (without postsharp) and it can be used in certain contexts to notify observers that a value of a property has changed. However, it still requires that each property actually broadcast the change notification - which generally requires it to be specifically coded to support that. Injecting change notification to existing code (without actually changing the source) is not an easy thing to do in straight-up C#. PostSharp (other other AOP/dynamic proxy libraries) make this sort of thing dramatically easier.
I'm not sure if you need to go down this road or not. But if you want to implement overrides of a method (i.e. generating new code for the method?) then it is possible with Emit. I would explore any other suggestions first before diving into those deep waters.
You're looking for INotifyPropertyChanged. You can dynamically implement that using PostSharp, Castle DynamicProxy or probably any other proxying library.
This does not solves the problem of having to add the tracking code dynamically, but can be interesting to read: Trackable Properties with Weak Events
With this stuff you are able to track changes to specific properties and it makes easier to implement INotifyPropertyChanged (i.e. track changes to all properties).
After doing extensive research on this subject, it appears that I can't do exactly what I'd like to do with .NET in its current state.
PostSharp's method is done at compile time, meaning I can't dynamically insert my own implementations to methods.
Reflection.Emit allows me to do this dynamically, but it generates a new instance of the created subclass - I need to do this so it works with the original instance.
INotifyPropertyChanging and INotifyPropertyChanged would be perfect if any of the existing .NET classes actually used them.
... so, at the moment I'm a bit stuck. I've put a more detailed piece on what I'm doing and how I'm trying to achieve in a post on my blog. Here's hoping .NET 4.0's dynamic dispatch will help!